The proposed research aims to elucidate the neural mechanisms of perceptual learning in the visual modality. Specifically, we aim to discover the loci of learning in the visual system and to describe the computational mechanisms whereby learning occurs. A significant body of research exists on the behavioral consequences of perceptual learning: typically, learning fails to generalize across various physical parameters of the stimulus. For example, if a perceptual discrimination is learned in one part of the visual field, the improvement does not transfer to other parts of the visual field. Thus, learning is assumed to stem from changes occurring in primary sensory mechanisms. However, there is a relative paucity of physiological evidence to explain these phenomena and the underlying neural mechanisms. We will use psychophysical methods in order to track the behavioral consequences of learning in two different visual discriminations, tapping different percpetual mechanisms. Then, we will use fMRI in order to measure learning-induced changes in the spatial extent of responses in visual cortical areas and in the tuning of large populations of cells in these areas. FMRI provides simultaneous recordings from mutliple brain areas. Therefore, it is a suitable method in order to track down the locus of learning in the visual system. Finally, we propose to explore the role of top-down modulation of Perceptual Learning by the cholinergic system. Animal studies show that activity in this system facilitates modality-specific and stimulus-specific perceptual learning. We will test the role of this system in human perceptual learning by administering a cholinesterase inhibitor commonly prescribed as treatment for Alzheimer's Disease (donepezil, trade name: Aricept) during training on a perceptual task. Percpetual training procedures have been suggested to provide health benefits in conditions as varied as dyslexia, amblyopia, congenital prosopagnosia and mild cognitive impairment in aging, but in most cases, the neural mechanisms underlying the training-induced improvements are unknown. Understanding the underpinnings of specific learning will allow the development of more effective clinical interventions in these and other conditions. Understanding the effects of cholinergic modulation on neural substrates of perceptual learning would shed light, not only on the functions of the cholinergic system in healthy individuals, but also on the role this system may have in cognitive disorders, such as Alzheimer's Disease.